Don't know why you're experiencing the problem you described, but I have been using Bryce for many years & have never moved objects in the way you described - because it requires holding down a key to control which axis to move along (which I find clumsy). Using the Repositon tool (circled in red) allows you to move objects much more conveniently & reliably, along any axis, by using the mouse alone.

"Every child is an artist. The problem is how to remain an artist once we grow up." - Picasso